Isolation of an IncP-1 plasmid harbouring mcr-1 from a chicken isolate of Citrobacter braakii in China

The plasmid-mediated colistin resistance gene mcr-1 has been found worldwide, but the diversity of organisms harbouring this gene is unknown. In this study, 12 colistin-resistant Citrobacter spp. isolates were obtained from diseased or dead chickens in China, and PCR analysis indicated that five were positive for mcr-1. One Citrobacter braakii strain (SCC4) with a multidrug-resistant phenotype was chosen for further analysis. SCC4 was resistant or intermediate-resistant to ten of the tested antibiotics, and the colistin minimum inhibitory concentration (MIC) was >4 µg/mL. A conjugation assay demonstrated successful transfer of colistin resistance to Escherichia coli strain J53 at a frequency of 10-7 cells per recipient cell. Whole-genome sequencing revealed that SCC4 contained 13 antibiotic resistance genes in its genome, and the mcr-1 gene resided on a 44-kb self-transmissible IncP-type plasmid of a recently discovered IncP-1 clade. In addition, the mcr-1 gene was part of an insertion element (ISApl1-mcr-1-orf-ISApl1) that was excised from the plasmid as a circular intermediate form. This is the first report of mcr-1-posiitve C. braakii of animal origin and these findings highlight the fact that the mcr-1 gene can be found in normal enteric flora as part of broad-host-range plasmids.